In general the absorption and retention of aqueous liquids, particularly body fluids such as urine, menses, etc., are accomplished by use of absorbent articles containing absorbent materials. Such articles include sanitary napkins, panty liners, interlabial devices, tampons, disposable diapers, incontinence pads, wound dressings, nursing pads, and the like. Generally, the most used absorbent materials are cellulose materials (e.g., defiberised wood pulp) and superabsorbent materials. In particular, when referring to disposable diapers or sanitary napkins and the like presently available in the market, the cellulose materials are typically in the form of bat or sheet, and generally further contain particulate absorbent materials, usually referred to in the art as superabsorbents or hydrogelling materials, which allow to manufacture thin but very absorbent core structures. A primary reason to incorporate superabsorbent material in particle form within an absorbent structure is its stabilization, in order to counteract the tendency of powdered material to bunch up or agglomerate, hence providing an uneven absorptive capacity in the absorbent structure, or also to dust off the structure itself. Known approaches are for example to adhesively fix the particles into a fibrous structure, or to disperse the powdered superabsorbent material in a fibrous matrix, e.g., cellulose pulp, and fix it in place mechanically, e.g., by calendaring or embossing. An alternative approach is to blend a superabsorbent particulate material into a thermoplastic matrix, e.g., a thermoplastic composition. The superabsorbent containing thermoplastic composition can be typically extruded or coated in any desired position and pattern onto a suitable substrate, to be then incorporated into an absorbent article, thus entirely providing the absorbent material in the article, or alternatively integrating a more traditional fibrous absorbent structure, with no risk of dust off of the particulate material, or displacement within the absorbent structure of the article. For example, EP 1013291 and WO 98/27559 describe a hot melt adhesive containing a superabsorbent polymer. WO 99/57201 illustrates compositions comprising a thermoplastic component and a superabsorbent polymer, the compositions in form of a film layer or applied to a disposable absorbent article with various hot melt adhesive application techniques. Applications WO 03/049777 and WO 04/028427 describe thermoplastic compositions comprising a matrix of a thermoplastic polymeric composition and superabsorbent particles dispersed therein, which have a particularly effective fluid acquisition and handling capacity, and absorbent articles comprising superabsorbent containing thermoplastic compositions arranged in a pattern of unattached spaced apart zones.
The technology of composite thermoplastic materials comprising a matrix of a thermoplastic composition and particles of superabsorbent material dispersed therein has provided a solution to the problem of particulate or powdered superabsorbent material “instability” within absorbent structures in absorbent articles, preventing particle displacement as, e.g., dust-off, agglomeration, or bunching up, and safeguarding the end users of absorbent articles virtually from any undesired contact with the superabsorbent particles upon normal use.
However, composite thermoplastic materials comprising particles of superabsorbent materials are relatively complex to formulate, and moreover usually imply rather high temperatures when they are applied onto a substrate in the molten state, since typically the thermoplastic matrix is a hot melt adhesive. Care must be taken since some substrates can be heat sensitive. Also relatively high temperatures of the thermoplastic matrix in the molten state could negatively affect the particulate material itself, hence already in the manufacturing process of the composite thermoplastic material, as well as upon subsequent application onto a substrate.
According to an alternative approach as disclosed in EP 157960, the superabsorbent particles may be dispersed into a liquid carrier, for example an oil carrier, and applied onto a substrate as a liquid dispersion. However, a liquid dispersion is generally not preferred for application onto substrates since it is not stable. It can in fact spread or move onto, e.g., film substrates, particularly on substrates which are not wetted by the liquid carrier, or alternatively be absorbed into porous, typically fibrous, substrates, such as for example nonwoven or cellulose layers, migrating within the substrate and impregnating it, thus likely modifying the characteristics of the substrate itself. Moreover, particularly after application onto a substrate the particles of superabsorbent material dispersed into the carrier can still move and provide a non-homogeneous distribution within the carrier, and also interfere during the application process, for example clogging the extrusion die or nozzle.
An improvement can be the use of a particulate material having low average particle size, which is advantageous not only in terms of better, e.g., liquid handling and absorption capacity of a liquid absorbing material comprising said particles dispersed into a carrier, due to the increased surface/volume ratio provided by smaller particles, but also in an easier processability of the liquid absorbing material, which may have lower viscosities when prepared and/or applied to a substrate. Moreover, smaller average particle sizes allow preparation of a more homogeneous dispersion in the carrier.
However, powdered materials, for example typically superabsorbent materials, also pose health risks to those involved in the manufacturing process, particularly when they are manufactured in very small average particle sizes as mentioned above. The finely powdered superabsorbent material can become airborne and can be inhaled by workers. Once inhaled, the superabsorbent material absorbs liquid within the respiratory passages swelling to many time its original size. This can result in blocked air passages and potentially traumatic health complications.
Hence there is the need for an improved material for liquid absorption which is simpler to produce and to apply. There is also a need for such a material that provides improved stability after application onto a selected substrate. There is also a need to provide such a material that has compatibility towards most substrates, including heat sensitive substrates and various particulate absorbent materials. There is also the need for a simpler process for making such a material which eliminates the health and environmental problems related to the handling of particulate absorbent material in very low average particle size.
Accordingly, it would be desirable to provide an improved liquid absorbing material comprising a particulate absorbent material dispersed into a carrier, which has better absorption characteristics, and an increased stability and compatibility with a wide range of substrates.
It would also be desirable to provide an improved process for making such a material.